The present invention relates to a method of temperature stabilizing an optical waveguide having a positive thermal optical path length expansion, in particular an optical fiber distributed feed back laser or a distributed Bragg reflector optical fiber laser.
The invention further relates to packaging of fiber lasers to reduce environmental influences, specifically such influences that act to reduce their performance. More specifically yet it relates to packaging techniques for fiber lasers that act to reduce frequency jitter and thus serve to create ultra narrow linewidth fiber lasers.
1. The Technical Field
It is well known in the field of optics that the performance of optical components depends on temperature via induced change in the optical path length. This dependence is due to a change of the refractive index (thermo-optic effect) and strain with temperature. Typically the thermo-optic effect yields the dominant contribution, and for most optical materials the thermo-optic coefficient is positive, i.e. the refractive index increases with increasing temperature. In silica this increase is of the order of +11xc2x710xe2x88x926/xc2x0 C. For components based on UV-written Bragg gratings in fibers or planar waveguides this results in a temperature drift of the center wavelength of approximately 0.01 nm/xc2x0 C. Although this figure is approximately 10 times better than what can be obtained in semiconductor based optical components it is still too high for a range of important applications. A notable example is found in optical communication systems based on dense wavelength division multiplexing where the channel spacing may be e.g. 100 GHz/0.8 nm and system administration requires a wavelength drift no higher than 0.001 nm/xc2x0 C., i.e. 10 times lower than the intrinsic value for UV-written Bragg gratings in fibers or planar waveguides. It is thus necessary to stabilize the wavelength.
Various methods of stabilizing the wavelength have been suggested in the art. In one method the temperature of the device is stabilized actively, e.g. by measuring the device temperature and controlling it through a suitable feedback. The disadvantage of this method is that energy is consumed which will dissipate to the rest of the system.
In other methods the thermo-optic coefficient is manipulated to balance the thermal expansion, or vice versa.
Generally, the temperature dependency of the center wavelength xcex of a Bragg grating in an optical fiber on temperature T is given by the following equation (1):                                           1            λ                    ·                                    ⅆ              λ                                      ⅆ              T                                      =                                            1              n                        ·                                          ∂                n                                            ∂                T                                              +          α          +                                    1              n                        ·                                          ∂                n                                            ∂                ϵ                                      ·                                          ∂                ϵ                                            ∂                T                                              +                                    1              Λ                        ·                                          ∂                Λ                                            ∂                ϵ                                      ·                                          ∂                ϵ                                            ∂                T                                                                        (        1        )            
where n, xcex1 and xcex5 are the values for the refractive index, the thermal expansion and the strain. xcex9 is the Bragg grating period. The 1st term including the thermo-optic coefficient       ∂    n        ∂    T  
represents the change in refractive index with temperature, the 2nd term represents the thermal expansion coefficient of the optical fiber, the 3rd term including the elasto-optic coefficient       ∂    n        ∂    ϵ  
represents the change in refractive index with strain, and the last term represents the change in the Bragg grating period with strain.
From this equation the following methods for temperature stabilization can be suggested:
The thermo-optic coefficient is changed to cancel out the contributions from thermal expansion and strain. In most fiber optical materials these two effects act together to increase the center wavelength with temperature. However, by tailoring the optical material to provide a negative thermo-optic coefficient, the positive contribution from the remaining terms is balanced to provide a stable center wavelength. The disadvantage of this method is that it is not easy to produce an optical material that provides a negative thermo-optic coefficient while maintaining other properties of the material.
Alternatively, the optical fiber can be mounted on a substrate under tension in such a way that its effective thermal expansion becomes negative to compensate the normally positive contribution from the thermo-optic and photo-elastic coefficients. When the optical fiber is mounted under tension the equation (1) reduces to equation (2):                                           1            λ                    ·                                    ⅆ              λ                                      ⅆ              T                                      =                                            1              n                        ·                                          ∂                n                                            ∂                T                                              +                      α            s                    -                                    1              n                        ·                                          ∂                n                                            ∂                ϵ                                      ·                          α              f                                                          (        2        )            
where xcex1s and xcex1t are the thermal expansion coefficients of the substrate and the optical fiber, respectively. The thermal expansion coefficient of the substrate can be made negative by two methods.
In a method, the substrate can be composed of two materials of different length and having different positive thermal expansion coefficients. The shortest piece of material is made from the material with the highest positive thermal expansion coefficient, and the longest piece is made from the material with the lowest positive thermal expansion coefficient. By fixing one end of the short piece to one end of the long piece, the other ends of the two pieces will approach each other as the temperature is increased. This presumes that the lengths and material parameters are balanced correctly. When an optical fiber is mounted under tension between these ends its effective thermal expansion becomes negative. A disadvantage of this method is that careful adjustments of the lengths and thermal expansions of the two pieces are required in order to ensure that the negative effective thermal expansion compensates the positive thermo-optic coefficient.
In another method, the substrate consists of a single material with an intrinsic negative thermal expansion coefficient. An optical fiber is mounted under tension on the substrate. By selecting and/or designing a substrate material with a suitable value of the negative thermal expansion coefficient, the effective negative thermal expansion compensates the positive contribution from the thermo-optic and photo-elastic coefficients of the optical fiber. This method has the advantage that once the correct material composition has been provided no further adjustments are required in order to achieve a stable center wavelength. Thus, this method has the advantage of simplicity in the mounting process; the exact length of the fiber is not important. Furthermore, depending on the substrate material the mount can be made considerably more robust.
Other than a change in center wavelength, temperature variatons also influence the spectral linewidth of optical waveguide lasers, e.g. optical fiber lasers. The spectral linewidth of lasers, including single frequency rare earth doped fiber lasers, is ultimately determined by optical spontaneous emission noise, corresponding to the Shawlow-Townes limit. For rare earth doped fiber lasers this lies in the Hz to sub-Hz region. In practical implementations, however, environmental effects will affect the cavity stability and lead to linewidths well above the Shawlow-Townes limit. Thus, although long term drift in temperature can be compensated by specialised packaging techniques such as those described above, small and rapid temperature fluctuations cause jitter in the center frequency. The frequency shift due to the thermo-optic effect is approximately 10-5xc2x0 C. xe2x88x921xc2x7xcexdxc2x7xcex94T Hz, where xcexd is the optical frequency and AT is the temperature change. As an example, if the frequency stability is required to be better than 1 MHz at 1550 nm, then the temperature fluctuations must be lower than 10xe2x88x923xc2x0 C. This way temperature fluctuations in the environment result in an increase in the effective linewidth. Another important contribution to jitter and linewidth increase comes from acoustic vibrations which affect the cavity via the elasto-optic effect. To stabilise the laser frequency and reduce its linewidth it is thus necessary to protect it from environmental influences. In doing so it is necessary to consider both acoustic and temperature effects, and with regard to the latter it is specifically necessary to consider rapid variations in temperature.
2. Prior Art Disclosures
Chu et al. xe2x80x9cMultilayer dielectric materials of SiOx/Ta2O5/SiO2 for temperature-stable diode lasersxe2x80x9d, Materials Chemistry and Physics, 42 (1995), pp. 214-216, discloses a SiOx/Ta2O5/SiO2 sandwiched waveguide design with an effective negative thermo-optic coefficient applied to temperature stabilizing diode lasers. Nothing is disclosed about temperature stabilizing optical waveguides.
U.S. Pat. No. 5,042,898 discloses a method wherein two pieces of different materials with different thermal expansion coefficients and different length are arranged to balance the thermo-optic coefficient of an optical fiber. This method has the disadvantage of requiring full control over the process used to fix the optical fiber to the substrate. Furthermore, the fiber is mounted suspended in the mount, which results in acoustic coupling and makes the packaging fragile. Finally, quartz is an ideal material candidate for the longer piece since quartz both has a very low thermal expansion coefficient and is cheap. However, quartz is also a fragile material.
International application WO 97/26572 discloses a method using a single substrate material with an intrinsic negative thermal expansion coefficient and a particular class of substrate material with intrinsic negative thermal expansion coefficient, lithium-alumina-silica type ceramic glasses heat treated to develop the beta eucryptite crystal phase. Beta eucryptite being a ceramic glass is potentially fragile. It exhibits thermal expansion anisotropy which results in microcracks. Patent Abstract of Japan Vol. 97, No. 6, abstract of JP-A-9 055 556 discloses a method of protecting an optical fiber against damage by coating a looped optical fiber and then encasing it by producing a resin coated sheet containing it.
It is the object of the present invention to provide a method of temperature stabilizing an optical waveguide having positive thermal optical path length expansion. in particular an optical fiber distributed feed back laser, or a distibuted Bragg reflector optical fiber laser, and thus to provide a robust temperature stabilized optical waveguide.
It is a further the object of this invention to provide a method for packaging optical fiber lasers so that they are protected from rapid environmental fluctuations such as those arising from small and rapid temperature variations or acoustic vibrations, thus producing optical fiber lasers with ultra narrow spectral linewidths.
This object is achieved by providing a method of temperature stabilizing an optical waveguide having a positive thermal optical path length expansion.
According to the invention, the method comprises affixing the optical waveguide to at least two points of a composite material having a negative thermal expansion; said composite material comprising a resin matrix having embedded therein fibers having a negative thermal expansion coefficient, and optionally fibers having a positive thermal expansion coefficient.
It is obtained that the negative expanding composite material compensates the positive contribution to the change in optical path length from the thermo-optic and photo-elastic coefficients of the optical waveguide. Furthermore, the composite material is easy to fabricate and exhibits high mechanical strength. This is very useful in construction of mechanical parts whereby a robust temperature stabilized optical waveguide is provided.
Also in case of both fibers of negative and positive thermal expansion coefficients being present, the negative thermal expansion of the final composite material can be accurately adjusted to a desired value.
Also, strong reinforcing fibers e.g. glass fibers can further improve the mechanical properties.
Fibers having a negative thermal expansion coefficient are known in the art. Suitable fibers are disclosed in U.S. Pat. No. 4 436 689, the content of which is incorporated by reference.
The object of packaging optical fiber lasers to produce ultra narrow spectral linewidth sources is achieved as stated in claim 22, namely by a method of packaging a fiber laser inside a matrix of curable viscous material that acts to dampen acoustic vibrations and temperature fluctuations.
This reduces the environmentally induced jitter of the laser and consequently reducing the spectral linewidth of the laser.
Fibers having a suitable negative thermal expansion coefficient can be used.
Generally, it is preferred that the fibers having a negative thermal expansion coefficient have a negative thermal expansion coefficient in the range from xe2x88x925xc2x710xe2x88x926/xc2x0 C. to xe2x88x9212xc2x710xe2x88x926/xc2x0 C., preferably from xe2x88x929xc2x710xe2x88x926/xc2x0 C. to xe2x88x9212xc2x710xe2x88x926/xc2x0 C.
The concentrations of fiber materials and resin matrix material are chosen to ensure that a composite material with the desired numerical value of the negative thermal expansion coefficient in order to compensate for the thermo-optic and elasto-optic coefficients is obtained.
It is preferred that the fibers are in an amount of 40 to 70% by volume.
The fiber materials may be present in any suitable form. Generally, it is preferred that the fibers are interwoven in the sence that fibers having different orientations are provided, which allows for longitudinal and transverse adjustment of the negative thermal expansion.
It is preferred that the composite material comprises 60% to 100% axially orientated fibers and 0% to 40% transversally orientated fibers.
Generally it is preferred that the fibers are arranged in a laminated structure of more than one layer whereby a particular good stability is obtained. Thus, for a given mechanical stability, a laminated structure including more layers allows for a higher percentage of fibers oriented axially whereby a numerically higher value of the negative thermal expansion coefficient is ensured. Other structures than laminated structures are possible.
Generally, fibers having a negative thermal expansion coefficient can be of any suitable material.
In preferred embodiments the fibers are fibers of materials selected from the group consisting of polyethylene, aramide, polyacrylate, polybenzobis-oxozole, polybenzobisthiazole, polyethylene naphthalene, polyethylene sulfide, polyamide-imide, polyether ether ketone, and polyethylene terephthaline, alone or in combination.
Polyethylene and aramide fibers are preferred. Particularly polyethylene fibers of the type Dyneema SK60, SK65, and SK66 and similar are preferred since these fibers have numerically high negative thermal expansion coefficients of about xe2x88x9212xc2x710xe2x88x926/xc2x0 C.
The resin matrix is any suitable resin matrix in which the fibers can be embedded with a suitable adhesion. In preferred embodiments the resin is a thermo-curing resin. It is preferred that the resin matrix is a consolidated matrix of epoxy resins, unsaturated polyester resins, vinyl ether resins, urethane resins and urethane acrylate resins.
In a particularly preferred embodiment the fibers are of polyethylene, especially those of the type Dyneema SK60, SK65, and Dyneema SK66, and the resin is an epoxy resin which is found most useful for these fibers.
Generally, a composite material having a negative thermal expansion according to the invention exhibits any desired negative thermal expansion coefficient. For the temperature stabilization of optical fibers, e.g. optical fibers with Bragg grating, it is preferred that the composite material exhibits a negative thermal expansion coefficient in the range from xe2x88x924xc2x710xe2x88x926/xc2x0 C. to xe2x88x9210xc2x710xe2x88x926/xc2x0 C., preferably in the range from xe2x88x926xc2x710xe2x88x926/xc2x0 C. to xe2x88x929xc2x710xe2x88x926/xc2x0 C.
Affixing of the optical waveguide to at least two points of the composite material having a negative thermal expansion can be established by any suitable method. E.g. affixing the optical waveguide to at least two points includes affixing the whole length of the optical waveguide.
In a preferred embodiment, a controlled tension is applied to the optical waveguide prior to affixing it to the composite material so that it is ensured that the thermal expansion of the waveguide is determined solely by the thermal expansion of the substrate and not by the thermal expansion of the waveguide itself over the temperature interval specified for the device.
Generally, any suitable optical waveguide can be temperature stabilized, e.g. single and multimode optical fibers.
In a preferred embodiment, the optical waveguide is an optical fiber, preferably a single mode fiber, the properties of axial symmetry and the flexibility of which make it particularly simple to temperature stabilize by affixing it to a composite material having a negative thermal expansion.
In another preferred embodiment, the optical waveguide is an optical fiber device, such as a reflection Bragg grating or notch filter, further preferably being polarization stable.
Particularly preferred optical waveguides include optical fiber lasers, preferably polarization stable, such as optical fiber distributed feed back lasers or distributed Bragg reflector optical fiber lasers, in particular rare earth doped optical fiber distributed feed back lasers having UV-induced Bragg gratings or rare earth doped distributed Bragg reflector optical fiber lasers also having UV-induced Bragg gratings. The rare earth dopants include the elements: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
Particularly preferred are stable polarization mode optical fiber distributed feed back lasers or stable polarisation mode distributed Bragg reflector optical fiber lasers. Stable single polarization mode operation of these devices is necessary for a number of important applications including optical communication where external modulation requires the use of polarization sensitive devices such as lithium niobate modulators.
In a preferred embodiment the optical fiber laser is spliced to a polarization maintaining fiber, and the polarization axes of the optical fiber laser and the polarization maintaining fiber have been aligned by twisting the fiber axes relative to each other prior to affixing both the optical fiber laser and the polarization maintaining fiber to the composite material having negative thermal expansion whereby the polarization extinction is optimized at the other end of the polarization maintaining fiber so that there is one predominant linear polarization.
The composite material having negative thermal expansion can be in any suitable form. In a preferred embodiment the composite material having negative thermal expansion is in the form of a tube or coating on the optical waveguide having a positive thermal optical path length expansion, whereby the optical waveguide affixed onto the interior part thereof is protected from external chock. Such a temperature stabilized optical waveguide is more compact; in particular in the specific embodiment of a fiber coating.
In another preferred embodiment, the composite material having negative thermal expansion is in the form of a substrate for the optical waveguide having positive thermal optical path length expansion whereby particularly simple standardized forms of the composite material having negative thermal expansion can be used to affix the optical waveguide to be temperature stabilized.
The invention furthermore provides a temperature stable, packaged DFB or DBR fiber laser. The fiber laser is mounted on a substrate material as described above with an intrinsic negative thermal expansion coefficient matched to balance the change with temperature of the refractive index of the fiber laser fiber.
With respect to packaging of fiber lasers to dampen thermal and acoustic fluctuations and so reduce the spectral linewidth, it is preferred that the laser be fixed in a curable viscous substance.
It is preferred that the cured substance in which the fiber laser is embedded has a high loss coefficient/dissipation factor in a wide range of vibration frequencies, specifically at acoustic frequencies. It is further preferred that the cured substance in which the fiber laser is embedded has a low thermal diffusivity so that transient heat flow is reduced.